[Haskell-cafe] [] vs [()]

Sam Danielson samdanielson at gmail.com
Fri Oct 10 01:17:31 EDT 2008

> What is the difference between empty list [] and list with one unit element [()]?

The [] constructor takes no arguments and is like Nothing in the Maybe
type. The list ":" (cons) infix constructor takes two arguments, an
element of type a and a list of type a, to construct a new list. Compare
to Maybe.

	data []    a = []      | a : [a]
	data Maybe a = Nothing | Just a

Another way of saying [()] is


which, comparing with the Maybe type, is similar to saying

	Just ()

but Just only takes one argument where (:) takes two.

Both List and Maybe are containers that have a null constructor, namely
[] and Nothing. "():[]" contains () similar to how "Just ()" contains
(). You can make your own list type and put () in it as follows.

	data List a = Empty | Cons a (List a) deriving (Show, Eq)
	() `Cons` Empty    -- This is ():[]


PS. I'm new on the list and also to Haskell.

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